Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a continuous or pulse mode.
Excimer lasers are a particular type of electric gas discharge laser and have been known as such since the mid 1970s. A description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled xe2x80x9cCompact Excimer Laser.xe2x80x9d This patent has been assigned to Applicants"" employer, and the patent is hereby incorporated herein by reference. The excimer laser described in Patent ""884 is a high repetition rate pulse laser. In FIG. 1, the principal elements of the laser 10 are shown (FIG. 1 corresponds to FIG. 1 in Patent ""884). The discharges 22 are between two long (about 23 inches) electrodes 18 and 20 spaced apart by about ⅝ inch. Repetition rates of prior art lasers, like the one described, are typically within the range of about 100 to 1000 pulses per second. These high repetition rate lasers are usually provided with a gas circulation system. In the above referred to laser, this is done with a long squirrel-cage type fan 46, having blades 48 as shown in FIG. 1 and in FIG. 2 which is FIG. 7 in Patent ""884. The fan blade structure is slightly longer than the electrodes 18 and and provides sufficient circulation so that at pulse rates between 100 to 1000 Hz, the discharge disturbed gas between the electrodes is cleared between pulses. The shaft 130 of fan 46 is supported by two ball bearings 132 as shown in FIG. 2A which is FIG. 9 of Patent ""884. The gas used in the laser contains fluorine which is extremely reactive. The fan rotor driving fan shaft 130 is sealed, within the same environmental system provided by housing structure members 12 and 14, by sealing member 136 as explained at column 9, line 45 of Patent ""884, and the motor stator 140 is outside sealing member 136 and thus protected from the corrosive action of the fluorine gas. However, bearing 132 is subjected to the corrosive action of the chamber gas as is the lubrication used in the bearing. Corrosion of the bearings and bearing lubrication can contaminate the gas.
Electric discharge gas lasers of the type described in U.S. Pat. No. 5,023,884 utilize an electric pulse power system shown in FIG. 3 to produce the electrical discharges, between the two electrodes. In such prior art systems, a direct current power supply 22 charges a capacitor bank called xe2x80x9cthe charging capacitorxe2x80x9d or xe2x80x9cC0xe2x80x9d 42 to a predetermined and controlled voltage called the xe2x80x9ccharging voltagexe2x80x9d for each pulse. The magnitude of this charging voltage may be in the range of about 500 to 1000 volts. After C0 has been charged to the predetermined voltage, a solid state switch 46 is closed allowing the electrical energy stored on C0 to ring very quickly through a series of magnetic compression circuits comprising capacitor banks 52, 62 and 82 and inductors 48, 54 and 64 and a voltage transformer 56 to produce high voltage electrical potential in the range of about 16,000 volts across the electrode which produces the discharge which lasts about 50 ns.
In prior art systems on the market the time between the closing of the solid state switch and the discharge is in the range of about 5 microseconds; however, the charging of C0 accurately to the preselected voltage has in the past required about 400 microseconds which was quick enough for pulse repetition rates of less than about 2,000 Hz. The reader should understand that accurate charging of C0 is very important since the control of the voltage level on C0 is in these systems the only practical control the laser operator has on the discharge voltage which in turn is the primary determiner of laser pulse energy.
U.S. Pat. No. 6,026,103 granted to some of the Applicants and others describe a gas discharge laser with a fan supported by roller bearings and magnetic axial positioning. In a preferred embodiment, the magnetic axial positioning was provided using a reluctance centering strategy in which the motor rotor was offset within the motor stator from its axially stable position. The embodiment described in detail in that patent included a ball and plate assembly at the non-drive end of the shaft for reacting the magnetic force created by the rotor offset. This laser was designed for repetition rates higher than the prior art 1000 Hz repetition rates. The squirrel cage-type fan blade was increased in size from 3.25 inches diameter to 5 inches and the fan was designed to operate at 5000 rpm as compared to 3000 rpm for a popular prior art design. Lifetime tests of the fan bearings described in the ""103 patent produced significant wear of the bearing plate at the non-drive (idle) end.
Prior art excimer lasers used for integrated circuit lithography typically require a system for cooling the laser gas which is heated both by the electric discharges and by the energy input through circulating fan discussed above. This is typically done with a water cooled, finned heat exchanger shown at 58 in FIG. 1.
When used as a light source for integrated circuit lithography excimer lasers, the laser beam parameters (i.e., pulse energy wavelength and bandwidth) typically are controlled to within tight specifications. This requires line narrowing of the laser beam (typically using a line narrowing module comprising a prism beam expander and a diffraction grating) and pulse-to-pulse feedback control of pulse energy and somewhat slower feedback control of wavelength.
What is needed is a better laser design for a pulse gas discharge laser for operation at repetition rates in the range of 4,000 to 6,000 pulses per second.
The present invention provides a gas discharge laser capable of operating at pulse rates in the range of 4,000 Hz to 6,000 Hz at pulse energies in the range of 5 mJ to 10 mJ or greater. Important improvements over prior art designs include: (1) a laser chamber having a gas flow path with a gradually increasing cross section downstream of the discharge electrodes to permit recovery a large percentage of the pressure drop in the discharge region, (2) a squirrel cage type fan for producing gas velocities through the discharge region of more than 67 m/s and capable of continuous trouble-free operation for several months, (3) a heat exchanger system capable of removing in excess of 16 kw of heat energy from the laser gas (4) a pulse power system capable of providing precisely controlled electrical pulses to the electrodes needed to produce laser pulses at the desired pulse energies in the range of 5 mJ to 10 mJ or greater at pulse repetition rates in the range of 4,000 Hz to 6,000 Hz or greater and (5) a laser beam measurement and control system capable of measuring pulse energy wavelength and bandwidth on a pulse-to-pulse laser with feedback pulse-to-pulse control of pulse energy and wavelength.